I liked Western Digital's My Cloud network-attached storage device when I reviewed it late last year, but relying on a single-drive NAS can be risky. If that drive fails, and you don't have a backup, you could lose all your data--forever.

I liked Western Digital's My Cloud network-attached storage device when I reviewed it late last year, but relying on a single-drive NAS can be risky. If that drive fails, and you don't have a backup, you could lose all your data — forever.

WD's My Cloud Mirror solves that problem by putting a second drive in the same enclosure, and configuring the drives as RAID 1. All the same data is written to both drives, so that if one drive fails, you can recover everything from the other.

The model reviewed here came with two 2TB drives, yielding 2TB of storage (not 4TB, because the drives are in RAID 1). There's nothing to stop you from reconfiguring the drives in RAID 0 for blinding speed and 4TB of storage, but that would throw your data-redundancy strategy right out the window. I wouldn't recommend that unless you're absolutely fastidious about backing up your NAS — and nobody is fastidious enough to avoid Murphy's Law.

The My Cloud Mirror will also let you back up its contents to another storage device via its USB 3.0 port, but an even better data security strategy would be to deploy a second My Cloud Mirror (or a My Cloud EX2 or EX4, but it must be a Western Digital device) at a remote location and back up the contents of each drive to the other (you can also do this over your local network, but that's not as safe as having backups at different physical locations).

If you don't want to go either of those routes, WD's software will let you back up your My Cloud Mirror to the cloud (using either your ElephantDrive or Amazon S3 account, though you'll need to pay for whichever service you choose). What it won't let you do is back up a client to the My Cloud Mirror itself over an Internet connection; the client must be attached to the same local network as the My Cloud Mirror.

Unlike a Dropbox account (or Connected Data's Transporter line), which maintain a folder on your local device that is synchronized with your cloud storage, files are stored only on the My Cloud device. The benefit to this approach is that you don't consume the limited storage on your device. The drawback is that you need to have Internet access to be able to retrieve your files.

As with the original My Cloud, Western Digital is marketing the My Cloud Mirror to consumers, and this box has most of the features that audience will want. There's an integrated FTP server, for instance, and peer-to-peer file-sharing (BitTorrent). Finally, there a number of apps you can run right on the box, including Joomla and WordPress, if you want to host your own website.